This case is use exactly like the English with. A final consonant will geminate; otherwise the val/vel is applied directly yo the trailing vowel.

Gemination

The most important thing about this case is the gemmination of a final consonant.
This means that you don not just put the -val/-vel ending onto the word:
you must gemminate the final consonant.
The -v- in -val/-vel changes into last consonant of the word.
The -val/-vel comes after other endings, so the last consonant may actually come from another ending.

Note that when the last consonant of a word is a diglyph then the first gylph of the letter doubles.

busz busz

utazom busszalI travel with (by) bus.

Take care with gemination because it may mean that you
do not recognise a word as having had a the instrumental case applied.
Learn to recognise -(2 consonants)el
and -(vowel)vel
at the end of words.